Heretofore, a fluoropolymer having sulfonic acid functional groups have been used as an electrolyte membrane for polymer electrolyte fuel cells since its ion conductivity, durability and heat resistance are excellent. A liquid composition in which such a fluoropolymer is dissolved or dispersed in a solvent, is used for forming an electrolyte membrane by a casting method. Further, the liquid composition is used as a coating agent for repairing the electrolyte membrane or for other applications.
As the liquid composition, a liquid composition has been known, which comprises from 2 to 18 mass % of a perfluorinated ion-exchange polymer (fluoropolymer) having sulfonic acid functional groups, and from 82 to 98 mass % of the following mixture (solvent) (Patent Document 1):
A solvent comprising from 20 to 90 mass % of water, and from 10 to 80 mass % of at least one member selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, n-butanol and 2-butanol.
However, such a liquid composition has following problems:
(1) Since the solvent contains a large amount of water, when the liquid composition is used for an application to contact with a metal device, metal corrosion tends to take place.
(2) In a case where the water contained in the solvent is reduced, if a shearing force such as stirring is applied to the liquid composition, the viscosity of the liquid composition becomes temporarily raised, and thus, handling efficiency tends to be poor at the time of preparation of the liquid composition or at the time of film-forming by a casting method.
(3) In a case where the water contained in the solvent is reduced, if the content of the fluoropolymer is increased, the viscosity of the liquid composition increases, and thus, handling efficiency tends to be poor at the time of preparation of the liquid composition or at the time of film-forming by a casting method.
Patent Document 1: JP-A-57-192464 (Claims)